A2 Biology: Sexual Reproduction in Plants

Revision notes on sexual reproduction in plants:

-Structure, cross/self-pollination, fertilisation, germination

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Angiosperms:- group of plants that produce flowers that contain sex organs.

Many common flowers contain both male and female sex organs = HERMAPHRODITE

Male gametes --> enclosed IN a pollen grain

Female gametes --> enclosed IN an ovule In an ovary

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Structure of an Insect-pollinated Flower


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Structure of an Insect-pollinated Flower

All Sepals = CALYX --> protects young flower bud

All Petals = COROLLA --> attracts pollinators with colour + shape

Receptacle = part of the stalk where other parts attach

Anther = produces pollen containing male gametes

Filament = supports anther

Stigma = receptive surface for pollen grains (sticky)

Style = joins stigma to ovary + pollen tube goes through it

Ovary = contains ovule + becomes the fruit

Ovule = carries female gametes + becomes seed on fertilisation

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POLLINATION = the movement of the pollen grain from the anther to the stigma of a plant of the same species

  • Self-pollination:- pollen on the stigma comes from a flower on the same plant or even the same flower
    • Reduced variation
    • Both gametes produced from meiosis of the same genome
    • Preserves genomes suited to environment
    • More likely to bring together unfavourable recessive alleles
  • Self-pollination better than nothing as it will produce more variation than asexual reproduction
  • Cross-pollination:- pollen on the stigma comes from a flower on a different plant of the same species
    • Meiosis in both plants produce haploid gametes
    • Greater variation
    • Through evolution - significant as some individuals will be more successful = NATURAL SELECTION

Mechanisms to ensure cross-pollination:-

  • Anthers + stigmas mature at different times (protandry + protogyny), anther + stigma at different levels
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Insect-pollinated Flowers

Usually large, brightly coloured + scented to attract insects.

  • Nectar is often produced to attach insects
  • Pollen grains are large, sticky + heavy, usually with rough surfaces so that they can stick to the insects bodies
  • Petals often enclose the stamens + carpels so insects rub against them when feeding
  • Stigmas are sticky so that pollen grains setting on them are not easily displaced
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Wind-pollinated Flowers

Often small + dull since they don't need to attract insects. No nectar or scent.

  • Lots of pollen produced as there is a lot wasted, never reaching a stigma
  • Pollen grains are small, dry, smooth + light, so that they are buoyant + easily blown about by air currents
  • Stamens usually have long, slender filaments that hang outside the flower to catch the wind, shaking out the pollen from the anther
  • Stigmas protrude + are large + feathery so that they provide a large surface area to catch pollen gloating in the air
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Male Gamete

Enclosed INSIDE a pollen grain

Each anther contains 4 pollen sacs - many pollen grains in each sac

DEHISCENCE = splitting open of the anther to release the pollen grains

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Female Gamete

Enclosed INSIDE ovule

Megasporocyte or megaspore mother cell divides by meiosis to form 4 haploid cells

  • 3 degenerate + the remaining one forms and embryo sac
  • The haploid cell within the embryo sac divides by meiosis 3 times --> producing 8 haploid nuclei
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The fusion of male (n) + female (n) gametes to produce a zygote (2n)

  • Male gametes IN pollen grain, female gametes IN ovary

Male gametes can ONLY reach the female gametes by means of a POLLEN TUBE

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Pollen Tube Growth

The male gamete must reach the female gamete.

  • Pollen grain lands on stigma - it absorbs water, swells + splits open = germination of pollen grain --> producing pollen tube
  • Pollen tube grows from the pollen grain, down the style (tube growth controlled by pollen tube nucleus)
  • Tube secretes enzymes which digest a pathway through the style
  • Once near the ovary the generative nucleus divides by mitosis --> forming 2 haploid nuclei
  • Pollen tube enters the ovule through the micropyle (the end of the tube and tube nucleus degenerate)
  • The tip of the pollen tube bursts open releasing the male gamete into the embryo sac + the two male nuclei enter
  • One of the male gametes fuses with the female egg nucleus to form the zygote, which will divide to form an embryo
  • The other male nucleus fuses with both polar bodies --> forming triploid endosperm nucleus (3n) - food store for developing embryo
  • Zygote becomes the embryo plant
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Seed Germination

Zygote --> embryo plant: consisting of

  • plumule - the future shoot
  • radicle - the future root

Integument --> testa (seed coat)

Micropyle --> remains where it is; allowing water to enter the seed during germination

Ovary --> becomes the fruit - a ripened ovary containing the seeds e.g. squash, tomatoes, beans, corn, grains

Cotyledons (seed leaf)

  • Monocotyledons --> only one cotyledon, food store surrounds the cotyledons e.g. cereal grains
  • Dicotyledons --> two cotyledons, food store absorbed into the cotyledons (contains endosperm) e.g. broad bean
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Seed Germination

Broad bean germination, requires:

  • Suitable temperature --> optimum temperature required for enzymes, required for germination (spring)
  • Water --> helps to transport material around the seedling e.g. nutrients
  • Oxygen --> Respiration - ATP energy for growth of the seedling
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